Study on Erosion and Siltation Change of Macrotidal Estuary in Mountain Stream: The Case of Jiao (Ling) River, China

Abstract

A macrotidal estuary with mountain-stream inputs (MEMSs) is characterized by strong hydrodynamic forcing, high turbidity, and complex channel morphology. This study combines field measurements (2005–2020) with a 2D hydrodynamic–sediment model to examine estuarine turbidity maximum (ETM) dynamics, erosion–deposition patterns, and the effects of engineering interventions in the Jiaojiang Estuary (JJE). Results show that the coupled influence of upstream floods and downstream macrotides produces highly seasonal and spatially variable water–sediment processes: mountain-stream floods exhibit sharp hydrodynamic fluctuations, and the estuary displays pronounced tidal-wave deformation. Over the 15-year observation period, the riverbed experienced alternating erosion (up to −3.5 m) and deposition (up to +4.2 m), with net erosion of 0.5–1.2 m occurring in most Ling River sections during high-discharge years. The ETM migrated about 30 km during spring tides, with near-bed suspended sediment concentrations reaching 50–60 kg/m³. Human activities—particularly historical sand mining—modified channel geometry and sediment composition, intensifying the exchange between bed material and suspended sediment and facilitating the formation and migration of the ETM. Extreme events further enhanced geomorphic adjustment: the post-Lekima (2019) flood produced maximum scour of −5.8 m in the upper Ling River and deposition of +3.2 m in the Jiaojiang main channel within weeks. Channel curvature and junction morphology strongly controlled flood-level distribution. Model experiments indicate that lowering shoal elevations and widening the cross-section at key constrictions can effectively reduce flood levels. Collectively, these findings clarify the morphodynamic evolution mechanisms of a MEMS system and provide quantitative guidance for flood-mitigation and estuarine-management strategies.

1. Introduction

Estuaries—semi-enclosed transition zones between land and sea—are among the most dynamic environments on Earth, shaped by the interplay of tides, river discharge, sediment transport, and morphological adjustment [1,2]. Tidal forcing and freshwater inflow jointly regulate essential processes such as salt intrusion, suspended sediment dynamics, turbidity structure, and bedform evolution, making estuaries natural laboratories for investigating hydrosedimentary interactions across temporal and spatial scales. Macrotidal estuaries, commonly defined by tidal ranges exceeding 4 m [3,4], display pronounced tidal distortion, strong current asymmetry, and vigorous sediment exchange. These characteristics exert decisive control over the formation and migration of estuarine turbidity maxima (ETMs), spatiotemporal variations in bed shear stress, and the patterns of erosion and deposition that underpin medium-term morphological evolution [4,5].

A distinct subset of these systems is the macrotidal estuary with mountain stream (MEMS) [6,7], which forms where steep, short, high-energy rivers meet strongly macrotidal coastal waters—conditions prevalent along the western Pacific margin, including southeastern China [7,8,9]. Mountainous rivers are characterized by steep gradients, rapid hydrological responses, and high sediment loads, particularly during extreme rainfall or typhoon events. When their flood-driven discharges interact with macrotidal dynamics, the resulting MEMS environment exhibits strong tidal–river flow competition, pronounced tidal asymmetry, frequent sediment resuspension, and complex cohesive sediment behavior, as well as rapid, multi-scale bedform adjustments [10,11]. These processes differentiate MEMSs from lowland estuaries and contribute to the formation of highly dynamic ETM zones shaped by tidal wave deformation, sediment flocculation, and nonlinear interactions between flow and morphology [4,6].

Recent studies in representative MEMSs [12,13,14]—such as the Jiaojiang, Oujiang, Minjiang, and other southeastern Chinese estuaries—have revealed significant seasonal and event-driven variability in ETM behavior, sediment concentration, and bed morphology [4,6,15]. Typhoon-induced floods, in particular, can trigger hyperpycnal flows, widespread sediment redistribution, and abrupt changes in channel geometry [16]. Although substantial progress has been made in describing suspended sediment transport, sedimentary textures, tidal asymmetry, and short-term morphological responses, much existing work remains geographically limited or event-focused. Basin-wide, medium-term analyses linking upstream hydrology with estuarine hydrodynamics and morphodynamics are comparatively scarce [17,18,19]. Furthermore, the combined influences of tidal wave distortion, bed shear stress variations, sediment resuspension, and flocculation on ETM evolution remain incompletely understood [16,20]. Human interventions—such as dredging, land reclamation, and channel modification—add additional layers of complexity, yet their quantitative impacts on hydrodynamics and sediment transport have not been systematically characterized [21,22]. Finally, the coupling among geomorphic changes, sediment dynamics, and ecological responses remains insufficiently explored despite its importance for sustainable estuary management [23,24].

Understanding the medium-term evolution of MEMSs requires continuous hydrological, sedimentary, and topographic records spanning decadal scales because morphological adjustments in macrotidal estuaries typically unfold over 10–30 years. Two decades of continuous data therefore provide an informative basis for analyzing basin-wide erosion and deposition, diagnosing the drivers of morphological change, and interpreting how MEMS systems respond to extreme events, sediment supply variability, and human activities [25,26].

The Jiao (Ling) River presents an exemplary setting for such an investigation [15]. Its steep upstream catchment, frequent typhoon-induced floods, and strongly macrotidal estuary create an ideal natural laboratory for examining the interactions among hydrodynamics, sediment transport, and morphological adjustment across multiple scales. Leveraging approximately 15 years of hydrological, sedimentary, and topographic data from the river’s upper reaches to its estuary, this study aims to: (1) elucidate the coupled dynamics of flood events, tidal asymmetry, and sediment transport in a mountainous macrotidal environment; (2) establish a basin-wide framework for analyzing medium-term erosion and deposition; (3) quantify the impacts of engineering interventions on hydrodynamics and sediment exchange; and (4) develop an integrated understanding of geomorphic evolution and system response in high-energy estuarine settings.

By addressing these objectives, this work advances the understanding of hydrodynamic–sediment–morphology coupling in MEMSs and provides a scientifically grounded basis for estuary management, flood mitigation, navigation planning, and ecological protection in regions shaped by the interplay of mountain streams and macrotidal forcings.

2. Materials and Methods

2.1. Study Area

The Jiao (Ling) River (JLR), located in Zhejiang Province in southeastern China, discharges into the East China Sea. It has a total length of 209 km. The upstream Yong’an Brook and Shifeng Brook converge at Sanjiang Village (SJV), forming the Ling River (LR). The LR extends approximately 46 km, with channel widths ranging from 200 to 1400 m. It has a natural drop of 1.6 m and an average bed gradient of 0.04‰, and is generally characterized as a typical mountain stream. Downstream, the LR joins the Yongning River at Sanjiangkou (SJK), forming the Jiaojiang River (JJR). The JJR is about 19 km long and 880–1800 m wide, with a natural drop of 0.4 m and an average gradient of 0.02‰. The Jiaojiang Estuary widens seaward and drains into Taizhou Bay (Figure 1). The combined effects of mountain-stream runoff and tidal forcing create highly complex morphodynamics in the estuarine reach, particularly between Miaolonggang (MLG) and Haimen (HM).

The Jiaojiang Estuary is influenced by an irregular semidiurnal tide. Tidal waves enter the system from Taizhou Bay and propagate landward through the trumpet-shaped estuarine channel. Upon entering the river, the tidal wave becomes increasingly modified by riverbed friction, the backwater effect produced by upstream runoff, and boundary-induced constraint and reflection. These factors intensify with distance upstream, leading to a shortened flood duration and an extended ebb duration, ultimately producing a markedly asymmetric tidal waveform. With further upstream propagation, the tidal signal gradually transitions toward a standing-wave regime.

Under typical hydrological conditions, the tidal current limit is located near SJV in the LR. During major flood events, however, this limit migrates downstream. Observations show that when the upstream peak discharge exceeds 2500 m³/s, the tidal current limit shifts approximately 20 km seaward. When the peak discharge surpasses 7500 m³/s, the flood tide vanishes throughout the estuary. In extreme floods, freshwater can advance as far as 25 km seaward from the river mouth.

Between 1959 and 2017, the recorded maximum tidal level in the JJR reached 5.64 m, while the minimum fell to −2.88 m, yielding an extreme tidal range exceeding 6 m. In general, tidal characteristics attenuate progressively from the estuary toward the upstream reaches. Rotational tidal currents dominate the outer estuary, transitioning to reciprocating flow within the JJR. Data collected from 2013 to 2017 indicate that flood current velocities in the Taizhou Bay–JJR–LR continuum are consistently higher than ebb velocities. The maximum depth-averaged flood velocities span 0.57–1.70 m/s, 1.57–2.30 m/s, and 1.42–2.28 m/s in the respective reaches, while ebb velocities range from 0.51 to 1.45 m/s, 1.01–2.00 m/s, and 1.42–2.28 m/s.

The combined influence of fluvial discharge and tidal flow enhances sediment resuspension within the channel, leading to the formation of a high-sediment-concentration region approximately 30 km from the estuary. This zone, commonly known as the estuary turbidity maximum (ETM), is primarily sustained by marine-derived sediment and resuspended material from the riverbed [27].

2.2. Data Collection and Methods

2.2.1. Data Sources and Survey Framework

This study uses hydrological and topographic data of the Jiaojiang River (JLR) collected between 2005 and 2020, including tidal levels, river discharge, suspended sediment concentration (SSC), salinity, and riverbed sediment properties. The river topographic dataset comprises 44 surveyed cross-sections from the upper reach near the Sanjiangkou junction to Baisha in the estuary (Figure 2 and

Table 1. List of Positions of Fixed-point Hydrological Survey Vertical Lines.

Sampling Vertical	Beijing Geodetic Coordinate System 1954 (Central Meridian 120° E)				Riverbed (Seabed) Elevation
	X (m)	Y (m)	B (° ′ ″ N)	L (° ′ ″ E)	National Vertical Datum 1985 (m)
1#	3,190,166	613,541	28 49 19.6	121 09 47.5	−6.4
2#	3,186,597	621,202	28 47 21.2	121 14 28.6	−7.7
3#	3,177,027	629,887	28 42 07.4	121 19 44.8	−5.2
4#	3,177,519	630,107	28 42 23.3	121 19 53.2	−4.1
5#	3,175,974	640,824	28 41 29.1	121 26 27.2	−9.6
6#	3,175,603	644,855	28 41 15.4	121 28 55.5	−6.4
7#	3,174,728	650,747	28 40 44.6	121 32 31.9	−6.0
8#	3,166,619	663,252	28 36 15.8	121 40 08.3	−6.0
9#	3,172,748	662,070	28 39 35.4	121 39 27.9	−5.4
10#	3,170,566	672,786	28 38 19.6	121 46 01.1	−10.7
11#	3,178,092	666,420	28 42 26.9	121 42 10.8	−4.9
12#	3,162,687	672,146	28 34 04.0	121 45 33.3	−10.7
13#	3,157,769	667,052	28 31 26.8	121 42 23.4	−8.7

). All hydrological measurements and topographic surveys used in this study—including those summarized in the subsequent tables—were provided by the Taizhou Port Authority (https://slj.zjtz.gov.cn/, accessed on 11 October 2025).

Survey years were not evenly spaced because field campaigns were arranged during key hydrodynamic or geomorphic periods, such as wet/dry seasons, post-flood adjustment stages, and major typhoon-induced events. Accordingly, datasets are interpreted as representative snapshots of hydrodynamic and morphodynamic states rather than continuous annual trends. Indicators were therefore computed separately for wet-season, dry-season, and post-flood conditions to capture the characteristic seasonal variability and extreme-event responses of a macrotidal estuary.

Field surveys were conducted in the hydrologically significant years 2005, 2013, 2014, 2016, and 2017. These campaigns correspond to major runoff or typhoon events and thus provide representative boundary conditions for short–medium-term morphodynamic evolution. Upstream discharge of the JLR varied between 20 and 12,000 m³/s during the study period, with the peak (12,000 m³/s) occurring during Typhoon Lekima in August 2019. Based on the surveyed topographic data, a digital elevation model (DEM) was constructed to quantify channel morphological changes. Human interventions—including channel regulation works, sand mining, and shoreline modification—were documented to support the interpretation of observed morphodynamic responses.

2.2.2. Instrumentation

A comprehensive suite of positioning, hydrological, and sediment-sampling instruments was employed during the surveys:

(1)

Positioning and Topographic Measurement

DGPS receivers (Trimble SPS351, sub-meter accuracy), 18 units, Made in America

Total stations (Topcon GTS-332N/GPT-3002LN, angular accuracy 2″ ± (2 + 2 PPm × D)), 5 units, Made in Japan

Digital levels (Trimble DiNi, DSZ1-class accuracy), 2 units, Made in America

(2)

Hydrodynamic Measurement

Automatic tide gauges (WSH, ±2 cm), 8 units, Made in China

Hydrological winches (HY-100, 100 kg), 20 units, Made in China

ADCPs (RDI, 600/1200/300 kHz, velocity accuracy 0.25 cm/s, heading accuracy 0.5°, max velocity 10 m/s, operating depth 40–70 m), 12 units, Made in America

ADPs (Nortek/Aquadopp, 1 MHz, velocity accuracy ±0.5 cm/s, heading accuracy 1°, depth 12–20 m), 2 units, Made in Norway

Lead weights (15, 30, 50 kg), 26 units, Made in China

Current meters (SLC9-2, speed range 0.05–3.5 m/s, speed error ±1.5%, direction error ±4°), 10 units, Made in China

(3)

Sediment and Grain-Size Measurement

Suspended-sediment samplers (XCL horizontal sampler, 2 L), 30 units, Made in China

Turbidity meters (OBS-3A, 0–4000 NTU/0–5000 mg L⁻¹), 8 units, Made in China

Bed-sediment grab samplers (5 kg), 18 units, Made in China

Laser particle-size analyzer (MasterSizer 2000, 0.02–2000 µm, DV50 accuracy ±1%), 1 unit, Made in China

2.2.3. Measurement Methods

(1)

Tide-Level Observation

Temporary tide gauges were installed two days prior to hydrological measurements and operated continuously for approximately two weeks. Measurements were taken hourly under normal conditions and intensified to every 5 min near high and low tides to accurately record tidal peaks and troughs. Concurrent data from long-term tidal stations were also collected for validation and calibration.

(2)

Fixed-Point Hydrological and Sediment Measurements

Thirteen verticals were established: five within the JLR channel (1#–5#) and eight in the estuary and nearshore area (6#–13#). Horizontal coordinates follow the 1954 Beijing Coordinate System (central meridian 120° E), and bed elevations follow the 1985 National Height Datum.

Each vertical was surveyed during large, medium, and small tidal ranges. A single tidal-cycle measurement lasted ~27 h to capture two complete tidal periods. Water depth, velocity (speed and direction), and SSC were recorded hourly and at 30 min intervals during flow reversals or peak velocities. Salinity and suspended-sediment grain size were sampled at four characteristic stages (flood-accelerating, slack-flood, ebb-accelerating, slack-ebb). Bed-sediment samples were collected once per tidal campaign.

Vertical sampling schemes:

• Depth > 4 m: six-point method (surface, 0.2 H, 0.4 H, 0.6 H, 0.8 H, near-bed);
• 2–4 m: three-point method (0.2 H, 0.6 H, 0.8 H);
• <2 m: single-point method (0.6 H);
• Salinity and grain size were sampled uniformly at 0.6 H.

(3)

Cross-Sectional Discharge Measurement

Discharge was measured at three representative cross-sections, corresponding to the 1#, 2#, and 5# verticals. Measurements were synchronized with the fixed-point hydrological surveys and taken every two hours from one hour before flood onset through a full tidal cycle.

2.2.4. Data Quality Assurance

All hydrological and topographic data were collected under a unified quality-control framework consistent with ISO 9001 [https://www.iso.org/standard/62085.html, accessed on 11 October 2025]. Instruments were verified before and after each survey, and representative devices (e.g., current meters) underwent laboratory cross-checks, showing maximum direction deviations within ±2° and meeting all technical specifications.

Field measurements followed a real-time validation procedure: tidal levels, velocity profiles, and SSC records were continuously checked for internal consistency, and any anomalies triggered on-site comparison using redundant instruments (e.g., ADCP versus mechanical current meter). Quality inspectors randomly reviewed vertical positioning and measurement operations, and each tidal-cycle dataset was audited immediately after collection.

Laboratory analyses were performed using calibrated instruments, with SSC, salinity, and grain-size tests conducted under standard protocols. All analytical results and processed hydrological indicators underwent multi-level internal review, including datum conversion checks and consistency screening. Only data that passed these audits were incorporated into the final dataset.

2.3. Numerical Simulation of ETM

The simulation of ETM in this study primarily relies on the work conducted by Zhang et al. [15].

2.3.1. Hydrodynamic and Sediment Transport Equations

In estuarine regions, two-dimensional tidal wave movement can be described by the following governing equations:

$${\displaystyle \frac{\partial Z}{\partial t}}+{\displaystyle \frac{\partial Hu}{\partial x}}+{\displaystyle \frac{\partial Hv}{\partial y}}=0$$

(1)

$${\displaystyle \frac{\partial Hu}{\partial t}}+{\displaystyle \frac{\partial Huu}{\partial x}}+{\displaystyle \frac{\partial Huv}{\partial y}}=-gH{\displaystyle \frac{\partial Z}{\partial x}}-{\displaystyle \frac{\partial H\overline{u^{\prime }{u}^{\prime }}}{\partial x}}-{\displaystyle \frac{\partial H\overline{u^{\prime }{v}^{\prime }}}{\partial y}}+{\displaystyle \frac{{\tau }_{sx}-{\tau }_{bx}}{\rho }}+fHv$$

(2)

$${\displaystyle \frac{\partial Hv}{\partial t}}+{\displaystyle \frac{\partial Huv}{\partial x}}+{\displaystyle \frac{\partial Hvv}{\partial y}}=-gH{\displaystyle \frac{\partial Z}{\partial y}}-{\displaystyle \frac{\partial H\overline{v^{\prime }{u}^{\prime }}}{\partial x}}-{\displaystyle \frac{\partial H\overline{v^{\prime }{v}^{\prime }}}{\partial y}}+{\displaystyle \frac{{\tau }_{sy}-{\tau }_{by}}{\rho }}-fHu$$

(3)

where $u,v$—flow velocity components in the $x$ and $y$ directions;

$Z$—water level;

H—water depth;

$f=2\omega \sin \varphi$—Coriolis parameter ($\omega$: Earth’s rotation rate);

$({\tau }_{sx}-{\tau }_{sy})$—wind stress on the water surface;

$({\tau }_{bx}-{\tau }_{by})$—bottom friction terms;

$g$—gravitational acceleration; $\overline{u^{\prime }{u}^{\prime }}$, $\overline{u^{\prime }{v}^{\prime }}$, $\overline{v^{\prime }{u}^{\prime }}$, $\overline{v^{\prime }{v}^{\prime }}$—Reynolds stresses.

The two-dimensional salinity transport equation is:

$${\displaystyle \frac{\partial HC}{\partial t}}+{\displaystyle \frac{\partial HuC}{\partial x}}+{\displaystyle \frac{\partial HvC}{\partial y}}={\displaystyle \frac{\partial }{\partial x}}(H{\varphi }_x,{\displaystyle \frac{\partial S}{\partial x}})+{\displaystyle \frac{\partial }{\partial y}}(H{\varphi }_y,{\displaystyle \frac{\partial S}{\partial y}})$$

(4)

where C is salinity; ${\varphi }_x,{\varphi }_y$ are diffusion coefficients in the $x$ and $y$ directions.

The two-dimensional non-equilibrium suspended-sediment transport equation is:

$${\displaystyle \frac{\partial HS}{\partial t}}+{\displaystyle \frac{\partial HuS}{\partial x}}+{\displaystyle \frac{\partial HvS}{\partial y}}={\displaystyle \frac{\partial }{\partial x}}(H{\epsilon }_x,{\displaystyle \frac{\partial S}{\partial x}})+{\displaystyle \frac{\partial }{\partial y}}(H{\epsilon }_y,{\displaystyle \frac{\partial S}{\partial y}})+\alpha \omega (S_{\ast }-S)$$

(5)

where S—suspended sediment concentration; ${\epsilon }_x,{\epsilon }_y$—sediment diffusion coefficients; $\alpha$—probability factor of sediment settling; $\omega$—sediment settling velocity; $S^{\ast }$—sediment-carrying capacity of the flow.

2.3.2. Solution Method

Based on coordinate transformation relations, the self-developed program [15] converts the governing equations from Cartesian coordinates into a general curvilinear coordinate system. The finite volume method is used for discretization of the governing equations. The water-level correction method is employed to solve the coupled water level–velocity field, and an Alternating Direction Implicit (ADI) method is used for solving the algebraic equation system.

2.3.3. Parameter Determination

(1)

Flow sediment-carrying capacity

Considering the characteristics of newly deposited fine mud on the riverbed, the sediment-carrying capacity is calculated using Luo Zhaosen’s empirical formula for fluid-mud environments [28]:

$$S_{\ast }=0.296{\gamma }_S{({\displaystyle \frac{\gamma }{{\gamma }_W}})}^{12.8}{\displaystyle \frac{U^2}{gH}}$$

(6)

where ${\gamma }_S$—specific weight of sediment particles; ${\gamma }_W$—wet bulk density of sediment; $\gamma$—water density; U—flow velocity.

(2)

Settling velocity of cohesive sediments

Suspended sediment in the Jiao (Ling) River and Taizhou Bay consists mainly of fine cohesive materials with high concentrations. Therefore, sediment settling velocity must account for flocculation effects, sediment concentration, and flow velocity. The settling velocity is calculated using the Cao Zude formula [29]:

$$\omega ={\omega }_0{K}_f{\displaystyle \frac{1+4.6S^{0.6}}{1+0.06V^{0.75}}}$$

(7)

3. Results

3.1. Morphology Change

Yong’an Creek, located in the upper reaches of the Ling River, has a total length of 144 km, a natural drop of 806.4 m, and an average gradient of 5.6‰. The Shifeng Creek is 134 km long, with a natural drop of 1125.6 m and an average gradient of 8.4‰. Downstream, the Ling River transitions into the Jiao River, which extends for approximately 19 km. This reach has a natural drop of about 0.4 m, an average bed slope of 0.02‰, an average channel width of roughly 1800 m, and typical water depths of 7–12 m. Overall, the upper JLR is representative of a steep-gradient mountain river, whereas the lower JLR gradually widens and shallows as it approaches the coast.

The lower reaches of the Jiaojiang River (JJR) form a pronounced trumpet-shaped estuary. Under an upstream discharge condition of 5620 m³/s, both river width and cross-sectional area decrease exponentially in the landward direction from the estuary mouth (Figure 3 and Figure 4). The thalweg longitudinal profile exhibits two distinct sedimentary shoals: one situated in the estuarine transition zone approximately 35 km from the mouth, and the other located near the outer estuarine sandbar (Figure 5).

Sediment in the JLR originates from both terrestrial and marine sources. Terrestrial sediment is primarily delivered by flood events and consists largely of coarse particles transported as bedload, resulting in a relatively limited overall contribution [30]. Historical records indicate that the annual average sediment load of the JLR is 1.22 million t/a [29]. During the dry season, sediment concentrations in the upper reaches are very low, averaging only 0.236 kg/m³. Concentrations increase modestly during the flood season, with the median grain size of suspended sediment ranging from 0.02 to 0.04 mm. During Typhoon Lekima in 2019, sediment concentration and discharge at two hydrological stations upstream of SJV exhibited synchronous variation. At peak flow, sediment concentrations reached 3.14 kg/m³ at Baizhi’ao Station and 2.14 kg/m³ at Shaduan Station (Figure 5). Terrestrial sediment deposited in the estuary is supplemented by finer silt and clay particles that are repeatedly resuspended and redistributed by tidal action [31,32]. Together, land-derived and marine-derived sediments shape the evolving bed morphology of the JLR.

During periods of strong tidal intrusion, marine-derived suspended sediment becomes the dominant component within the estuary, driven landward by flood-tide currents. In contrast, during high-runoff conditions—such as wet-season discharge peaks or typhoon-induced floods—terrestrial inputs dominate the sediment supply. This shift highlights the temporal alternation of prevailing sediment sources under varying hydrodynamic regimes. Average suspended sediment concentrations in the estuary range from 4 to 8 kg/m³, while near-bed concentrations can reach up to 50 kg/m³. Marine-sourced sediment is partly supplied by coastal currents originating from the Yangtze River to the north, and partly by the combined action of wave-induced resuspension and tidal-current transport [33]. Based on hydrological observations from December 2013, the suspended sediment transported through the HM section during a single spring tide is estimated at approximately 1.4 million tons—exceeding the average annual sediment load of the entire JLR basin.

3.2. Hydrodynamic

3.2.1. Runoff

The runoff in the JLR Basin is mainly derived from precipitation, and its annual distribution is basically synchronized with precipitation. The catchment area of JLR basin is small, the annual maximum discharge is often several times the average discharge, and the runoff process presents the characteristics of surge and sudden fall, which is a typical characteristic of mountain stream rivers [34,35,36]. Due to the influence of plum rain and typhoon rain, the runoff in the flood season (April–September) accounts for 75% of the total value; the dry season (October to March) accounts for only 25% of the annual value (Figure 6 and

Table 2. The natural annual runoff of different reaches in JLR (unit: ×100 million m³).

Reaches	Annual Average Value	Annual Runoff of Different Frequencies
		50%	75%	90%	95%
Yong’an Creek	22.78	28.72	21.95	17.37	12.02
Shifeng Creek	11.76	15.26	11.21	8.55	5.52
Ling River	14.11	18.38	13.42	10.16	6.49
Jiao River	22.14	28.36	21.15	16.40	11.01
Total	70.79	90.71	67.73	52.48	35.04

). The flood peak in the flood season is characterized by rapid rise and fall and generally lasts for 1~2 days. The annual average runoff of JJR is nearly 163 m³/s. The maximum flood peak discharge recorded in JJR was 16,300 m^3/s, and the minimum discharge was only 0.39 m³/s. The ratio of the annual maximum peak discharge (6195 m³/s) to the annual minimum discharge (4.19 m³/s) can reach 1480 times.

3.2.2. Tide

During the dry season, hydrodynamics in the JJR are dominated by tidal currents, which exhibit a twice-daily fluctuation. During the wet and flood seasons, river discharge becomes the controlling factor. Owing to the strong contrast between high-flow and low-flow conditions, the intensity of tidal motion throughout the JLR varies substantially (

Table 3. The tidal range in each tide station.

Sites	Mean Range of Tide (m)					Maximum Tidal Range (m)				Minimum Tidal Range (m)
	2005 *	2013 *	2014 *	2016 *	2005 *	2013 *	2014 *	2016 *	2005 *	2013 *	2014 *	2016 *
XM *	5.65	5.54	4.84	5.23	6.31	6.61	6.85	6.78	4.77	4.41	1.25	3.43
MLG *	6.27	5.14	4.90	4.93	6.75	6.39	6.73	6.55	5.85	3.95	2.98	3.03
XA *	6.04	4.87	4.74	4.80	6.47	6.11	6.54	6.54	5.65	3.67	3.07	2.82
SXF *	5.38	4.55	4.46	4.48	5.85	5.83	6.20	6.23	4.96	3.37	2.77	2.63
HM *	5.02	4.26	4.23	4.05	5.63	5.61	6.00	5.84	4.24	3.02	2.44	2.32

Notes: * XM: Ximen, MLG: Miaolonggang, XA: Xi’Ao, SXF: Shixianfu, HM: Haimen; 2005 *, 2013 *, 2014 *, 2016 *: Measurement time periods, corresponding to 3–4 September 2005; 29 November–13 December 2013; 1–30 June 2014; 5 December 2016–4 January 2017.

). In the dry season, flood currents encounter incoming tides in the upper reaches, and tidal influence can extend 6–10 km into the tributaries upstream of SJV. During flood periods, the flood-induced tidal boundary can shift directly to the estuary mouth. When upstream discharge reaches 2500 m³/s, the tidal current limit moves to approximately 20 km seaward of the estuary. When discharge exceeds 7500 m³/s, freshwater can extend up to 25 km beyond the estuary, resulting in the disappearance of the flood tide within the estuary.

The reach downstream of SJK in the JJR is characterized as a strong-tide zone. The average annual tidal range at the Haimen (HM) station is 4.0 m, with a maximum annual range of 6.3 m. Prior to the period of intensive sand mining (1958–1980), the average tidal range at HM was about 4.0 m, compared with approximately 3.5 m at Ximen (XM) [37]. Although the tidal range at XM was smaller, both the mean high tide and mean low tide levels were higher than those at HM. From 1980 to 2005, extensive sand mining lowered the riverbed by 2–4 m, strengthening tidal currents, reducing low-tide levels along the channel, and producing a marked increase in tidal range. By December 2016, the tidal range at HM was 4.05 m, while the tidal range at XM had increased to 5.23 m.

3.2.3. Tidal Current

The tidal current outside the estuary exhibits a rotating flow pattern. The tidal current enters the main channel, where the river flow becomes predominantly reciprocal. The intensity of runoff significantly affects the tidal current in the JJR. As upstream flow increases, flood current velocity decreases while ebb current velocity increases within the estuary. The tidal current in the JLR typical experiences due to the influence of channel topography, with the flood discharge exceeding that of the ebb tide. Figure 7 and Figure 8 illustrates that the average flood current velocity in the JJR ranges from 0.57 to 1.70 m/s, while the ebb current value ranges from 0.51 to 1.45 m/s. The maximum average vertical flood velocity is about 1.57~2.30 m/s and the ebb value is about 1.01~2.00 m/s. The maximum average vertical flood current velocity is 1.42–2.28 m/s near MLG and XA of LR, and the ebb current velocity is 1.33–1.90 m/s.

3.3. Sediment

3.3.1. Sediment Concentration

As described earlier, sediment in the estuary includes contributions from both land-based and marine sources. Specifically, the upstream sediment contribution is relatively minor. Historical data indicates that the average sediment load of the JJR is approximately 1.22 million t/a, with an average sediment content of around 0.236 kg/m³. (https://slj.zjtz.gov.cn/, accessed on 11 October 2025).

The variation in sediment concentration along the JLR is shown in Figure 9. During the representative period 2013–2014, the average sediment content in the JLR consistently exceeded 3.0 kg/m³, while the average sediment concentration in Taizhou Bay remained below 3.0 kg/m³. Generally, the sediment concentration decreases gradually from the upper reaches of the JLR to the estuary. In the upper reaches of LR, the maximum average vertical sediment concentration reaches approximately 18 kg/m³, while in the JJR it is nearly 10 kg/m³. The average vertical sediment content near the estuary is commonly less than 5 kg/m³, and the average value in Taizhou Bay is less than 0.2 kg/m³.

3.3.2. Sediment Properties

The suspended sediment in JLR has a median particle size of is 0.005–0.0102 mm, as indicated by the sediment properties. The particle size of suspended sediment progressively decreases from LR to JJR. Figure 10 illustrates that the sediment in LR has greater thickness compared to JJR. A slight discrepancy in particle size exists between the suspended sediment in the JJR. This suggests a higher frequency of exchange between the two sediment types.

3.4. Salinity

The change in salinity can partially characterize one of the important hydrological factors affecting hydrodynamic exchange in estuarine areas. As the tide rises, saltwater infiltrates and mixes with fresh water, resulting in a different distribution of tidal current vertical velocity and sediment concentration compared to that of general open channel water [38,39,40]. This, in turn, affects the process of riverbed erosion and deposition in this area. The intensity of mixing serves as a crucial indicator for estuary classification. In JJR, the salinity distribution is lower within the estuary, even lower upstream, and higher outside the estuary [26,38,39,40]. During the dry season, the runoff is minimal, leading to a strong mixing effect. The average salinity in JJR ranges between 11.1‰ and 18.0‰, while it reaches between 19.9‰ and 28.6‰ in the estuary and outside. Based on the discriminant mixing intensity index proposed by H.S. Simons, the JJR estuary can be classified as a strongly mixed estuary [36]. During flood and ebb tides, the saltwater moves back and forth in the estuary resembling a ‘piston’. Longitudinally, there is a clear density gradient in the salinity ranges from 3‰ to 10‰, which represents the optimal range for flocculation. Flocculation of fine sediment benefits the circulation and retention of sediment in the estuary, ultimately increasing the sediment concentration indirectly [41].

3.5. Motion Characteristics of ETM

The motion process of ETM observed during the spring tide in December 2013 (the most complete dataset for ETM motion) is depicted in Figure 11. It shows that in the dry season, the average vertical sediment concentration in LR can exceed 20 kg/m³, and the maximum value at the bottom is nearly 60 kg/m³, which appears in the ebb tide period. The sediment content within the JLR is generally higher than outside the estuary. Under the encounter of upstream runoff and downstream tidal current, the motion characteristics of ETM are gradually formed in the river channel.

The ETM undergoes downstream motion during ebb tide and upstream motion during flood tide. The swing range of ETM can reach up to 30 km during spring tide, primarily extending from Niutoujing (NTJ) in JJR to Xi’ao (XA) in LR. The bottom value is larger than that of the surface value based on vertical distribution. The sediment concentration is high during the flood season, while the vertical gradient is small.

During ebb tide, as the dynamic conditions weaken, sediment gradually settles, resulting in a significant rise in sediment concentration near the bottom and the formation of floating mud (Figure 12).

4. Discussion

4.1. Riverbed Morphological Characteristics and Evolution Process

The Jiaojiang River features a well-developed trumpet-shaped estuary in which both channel width and cross-sectional area decrease exponentially in the landward direction (Figure 4). The thalweg longitudinal profile shows two prominent sedimentary uplifts: an inner uplift located approximately 35 km from the estuary mouth within the transition zone, and an outer uplift at the estuarine sandbar (Figure 5). This dual-uplift configuration represents a characteristic morphological signature of macrotidal estuaries influenced by mountain streams (MEMS), reflecting the combined effects of sharp-peaked mountain flood runoff and strong tidal forcing. Such morphology distinguishes MEMS from purely tidal-dominated systems (e.g., the Severn Estuary) and from runoff-dominated estuaries (e.g., the upper Yangtze Estuary) [37].

A comparison of the 2013 and 2016 bathymetric surveys (Figure 13) reveals a consistent spatial pattern of alternating erosion and deposition governed by several key nodal sections (MLG, XA, SXF, SJK, HM). These nodes coincide with pronounced channel bends or bedrock-controlled constrictions that modulate local hydrodynamics by regulating flow acceleration, deceleration, and redistribution. Between the two survey years, high-discharge events during the flood season generated net erosion throughout most reaches of the Ling River and Jiaojiang River, with typical scour depths ranging from 0 to 1 m. In contrast, tide-dominated dry-season conditions promoted localized siltation in low-energy zones.

These nodal controls arise because sharply curved or constricted reaches impede both upstream and downstream sediment transport. When flood flows encounter a curved boundary, flow velocity decreases, producing helical circulation and backwater effects that favor deposition immediately upstream of the node. Conversely, constricted cross-sections limit tidal intrusion during the flood season and enhance ebb-dominated sediment export during the dry season. This bidirectional sediment-blocking mechanism, together with tidal asymmetry, produces the characteristic “erosion–siltation–erosion” segmented pattern and exerts a dominant influence on the medium-term morphological evolution of the JLR bed. Comparable nodal effects have been documented in the Qiantang River Estuary and the Modaomen Estuary of the Pearl River; however, in MEMS systems these controls are markedly amplified by the extremely sharp flood hydrographs (rise and fall within ~48 h), rendering riverbed adjustment strongly event-driven rather than gradual.

4.2. The Impacts of Sand Mining

Historical sand mining (1980–2005) substantially altered the morphology of the JLR, causing 2–4 m of artificial bed deepening and enhancing tidal propagation in the lower reach, as documented by the Taizhou Port Authority. These long-term disturbances shaped the pre-2005 morphological baseline but fall outside the quantitative scope of this study, which focuses on short–medium-term evolution during 2005–2020 based on directly measured hydrological and topographic datasets.

Because systematic before–after monitoring data on mining intensity, spatial extent, and morphological response are unavailable for the full historical period, the effect of sand mining cannot be incorporated into the quantitative analysis. Instead, it is referenced only as background information to explain why the contemporary riverbed—after cessation of large-scale mining—shows reduced scouring–silting amplitude and has approached a state of dynamic adjustment. A detailed assessment of mining-induced geomorphic change is beyond the data coverage of this study and is therefore identified as a topic for future work.

4.3. Relationship Between ETM and Estuarine Geomorphology

Two distinct morphological protrusions are evident in the lower reaches of the JJR. One extends approximately 35 km within the estuarine transition zone, from MLG to HM, while the other is an offshore sandbar located seaward of the estuary mouth. The behavior of the estuarine turbidity maximum (ETM) in macrotidal estuaries influenced by mountain streams (MEMS) differs fundamentally from that in slowly mixed estuaries, both in origin and depositional pattern. In slowly mixed estuaries, the ETM is commonly positioned near the stagnation point and the apex of the salt-wedge front. It becomes particularly pronounced during the flood season and typically forms near estuarine sandbars, as observed in the Yangtze River Estuary.

In contrast, the ETM in MEMS exhibits pronounced oscillatory movement within the estuary. When runoff decreases below the annual mean, the stagnation point shifts landward, often far upstream of the estuary mouth. Consequently, the ETM position is not aligned with the sandbar but instead corresponds more closely to the saltwater intrusion limit during the dry season. Within this dynamic zone, two principal sedimentation areas develop: one in the estuarine transition section and another in the flow-diffusion zone outside the estuary. A comparable pattern has been documented in the Hudson Estuary [1].

The ETM acts as a major sediment reservoir for the estuarine transition, supplying more material than either riverine inputs or tidal transport alone. Sediment within the ETM undergoes bidirectional exchange during flood and ebb tides, and repeated cycles of suspension, deposition, and resuspension generate intense erosion–deposition alternation within the estuary. During the dry season, the ETM shifts upstream with the flood tide, enhancing bed-marking processes in the transition zone. During the flood season, sediment stored in the ETM—together with coarser material supplied from the upper reaches—is exported to the outer depositional zone, contributing to the formation and evolution of the seaward sedimentary bulge.

4.4. Topographic Evolution Under Complex Hydrodynamic Conditions

Flood events in the JJR basin are overwhelmingly associated with typhoon activity. Between 1949 and 2020, an average of 3.3 tropical cyclones affected the basin each year, with a maximum of seven events recorded in both 1961 and 1981. Typhoons occur most frequently from July to September and typically persist for 2–4 days, though some last as long as 6 days. During these events, storm surge at HM can exceed 100 cm. Typhoon-generated floods in the region are characterized by a sudden rise and rapid fall in discharge. Because the lower JJR has a gentle longitudinal slope, its flood response is comparatively slower, and tidal influence remains strong throughout the rising and falling stages, exerting significant control on bed-form development.

Typhoon “Lekima” on 10 August 2019 provides a representative example. The peak discharge of the JLR reached 14,000 m³/s (Figure 14), and the combined effects of storm surge and tidal forcing elevated the water level at XM to 10.78 m—the highest flood peak and water level on record. Figure 15 shows the morphological adjustments in various JJR sections before and after this event. Prior to the typhoon, in July 2019, riverbed elevations displayed minimal variability, indicating dynamic equilibrium. During the passage of Lekima, pronounced scour developed between SJV and YQ in the LR, primarily due to the sharp, high-magnitude flood pulse. Coarser sediment mobilized from the upper reaches was subsequently transported into the JJR, producing the highest average bed elevation within this reach.

Following the typhoon, tidal currents regained dominance in the lower river. Sediment deposited during the flood was progressively advected upstream by the flood tide, generating notable re-siltation within the LR from August to October 2019, while the JJR experienced net erosion during the same period. By June 2020, before the onset of the next flood season, the JJR riverbed had largely returned to a state of dynamic equilibrium.

Overall, the approximate boundary between flood-driven scour and post-event siltation is associated with Lekima lay between YQ and SJK. Upstream of this zone, the channel is relatively narrow and deep, leading to larger fluctuations in bed elevation. Downstream, toward the estuary, the channel widens and shallows, and morphological responses are more muted (Figure 15). This contrast highlights the strong spatial variability in typhoon-driven adjustment within the JJR system.

Riverbed erosion and siltation are closely linked to changes in water and sediment conditions. The transport of sediment by runoff and tidal currents creates a dynamic equilibrium stage in the evolution of the riverbed. As upstream flood peak flow increases, both the upstream scour area and downstream siltation area of the riverbed gradually expand downstream. During the dry season, tidal current controls sediment movement in the LR, causing downstream sediment deposits to be redistributed upstream (Figure 16 and Figure 17 and

Table 4. Variation rate of each section area before and after the ‘Lekima’.

Sections	2018.12~2019.4	2019.7~2019.8	2019.8~2019.10	2019.10~2020.6
SJV	59.3%	12.4%	−10.7%	−22.5%
WJM *	9.4%	18.1%	−8.5%	−20.1%
MLG	−20.8%	22.6%	−18.2%	−19.3%
PKC *	−6.7%	19.5%	−4.4%	−4.9%
YQ *	−2.7%	0.7%	4.3%	−1.9%
SJK *	1.8%	−3.1%	0.7%	2.6%
HM	0.0%	−5.7%	8.5%	−1.0%
NTJ *	−12.5%	−5.6%	5.4%	4.3%

Notes: * WJM: Wangjiangmen; PKC: Pukoucun; YQ: Yongquan; SJK: Sanjiangkou; NTJ: Niutoujing.

).

4.5. Causes of Flood Disaster and Flood Control Measures

According to hydrological observations during the flood induced by Typhoon Lekima, the flood-affected reach in Linhai City extends approximately 8.3 km, with a water-surface drop of 3.99 m and an average hydraulic gradient of 0.48‰. The narrow and highly sinuous channel, combined with bridges, exposed bedrock, and substantial flow resistance, produces river friction far greater than that of other reaches. The flood characteristics of a mountain–estuary–mixed system (MEMS) are tightly linked to channel morphology and tidal processes. In the JLR–JJR system, the spatial position of key bayonet nodes and the curvature of the channel jointly shape the longitudinal distribution of water levels.

The upstream portion of the JJR meanders within a confined valley flanked by steep mountains, creating a sequence of natural bayonet nodes. These constrictions induce uneven upstream flood routing. The pronounced longitudinal slope accelerates flood confluence. Meanwhile, urbanization and riverbank encroachment caused by embankments further restrict the effective cross-sectional area, resulting in a sharp rise in flood peaks and prolonged duration of water levels exceeding safety thresholds.

Downstream, the riverbed becomes comparatively flat and opens into a trumpet-shaped estuary. Strong tidal forcing elevates water levels and slows flood discharge. During Lekima in 2019, extreme rainfall generated a flood in the Ling River with a recurrence interval of roughly 20 years, causing extensive inundation in Linhai City. The existing embankment system proved insufficient to withstand a flood of this magnitude. This underscores the necessity of implementing targeted channel regulation measures in both the upstream and downstream reaches that are prone to flooding.

To mitigate future extreme floods, engineering interventions such as dredging and bank cutting have been evaluated to enlarge the hydraulic conveyance of the channel. Figure 18 presents three alternative designs for increasing the cross-sectional area of the LR. Scheme 1 dredges a 15 km upstream curved reach to an elevation of −6 m with a bottom width of 60 m. Scheme 2 removes lateral shoals that obstruct flow over the same 15 km reach, lowering the shoal elevation to −4 m. Scheme 3 proposes cutting and straightening a key bayonet node to form a new spillway branch 280 m wide at an elevation of −5 m. Scheme 4 is a comprehensive plan integrating Schemes 1–3.

Table 5. Changes in the highest flood level after the implementation of each project plan during ‘Lekima’ (Unit: m).

Section	Scheme 1	Scheme 2	Scheme 3	Scheme 4
S1	−0.239	−0.620	−0.075	−0.782
S2	−0.330	−0.799	−0.079	−0.964
S3	−0.143	−0.728	−0.089	−0.913
S4	−0.162	0.151	−0.101	−0.021
S5	−0.155	0.017	−0.108	−0.150
S6	−0.036	0.013	−0.111	−0.193
S7	−0.032	0.010	−0.121	−0.221
S8	−0.029	0.006	−0.143	−0.124
S9	−0.014	0.005	−0.161	−0.162
S10	−0.005	0.003	−0.212	−0.216
S11	−0.004	0.001	−0.178	−0.176
S12	−0.004	0.000	−0.136	−0.135
S13	−0.002	0.000	−0.092	−0.100
S14	0.000	0.000	−0.052	−0.053

summarizes the reduction in flood levels under each scheme. Schemes 1 and 2 both expand the cross-section along the main river course, while Scheme 3 focuses on localized improvement at a critical bend. The combined dredging–bank-cutting strategy delivers greater benefits than localized intervention alone, because enlarging the cross-section continuously along the river enhances overall conveyance capacity. Scheme 1 widens the channel bottom, whereas Scheme 2 targets shoals and bayonet zones along the banks; the latter proves more effective than bottom dredging alone. Both schemes show increasing flood-level reduction effects toward the upstream direction.

These results demonstrate that MEMS flood levels are governed primarily by the geometry of key bayonet nodes and the degree of channel curvature. High shoal elevations narrow the effective cross-section, and constricted bayonet nodes raise flood levels significantly. Lowering shoal elevations and widening water-blocking bayonet zones yield the most substantial flood-level reductions. Consequently, integrated multi-scheme engineering measures are strongly recommended for robust flood mitigation.

5. Conclusions

The hydrodynamic and sedimentary characteristics of the JLR riverbed were examined using hydrological observations and previous numerical simulations. The JLR represents a typical mountain stream influenced by macrotidal processes. The main conclusions are as follows:

(1)

The interaction between river runoff and strong tides produces highly variable hydrodynamic conditions, leading to complex sediment transport patterns and pronounced riverbed evolution. Marked seasonal differences occur: erosion dominates in wet years when the average annual discharge exceeds 164 m³/s, while deposition is more common in dry years.

(2)

Human interventions—especially sand mining—exert substantial influence on erosion and deposition patterns within the JLR. Such activities can drastically alter channel morphology and modify riverbed sediment composition, triggering abrupt shifts in local hydrodynamics and sediment transport.

(3)

The development of the estuarine turbidity maximum (ETM) in macrotidal estuaries is controlled by tidal deformation and sediment supply. Tidal deformation drives upstream transport of suspended sediment through Stokes drift, while the abundance of fine particles in the riverbed provides sufficient material for ETM formation.

(4)

The ETM position in a macrotidal estuary differs fundamentally from that in slow, weakly tidal estuaries. In the latter, the ETM typically coincides with the stagnation point, the salt-wedge tip, or a sandbar near the estuary mouth. In the MEMS, however, the ETM forms mainly during spring tides, with its active zone and the downstream diffusion zone representing two distinct sedimentary processes near the estuary.

(5)

Erosion and deposition in a MEMS are governed jointly by water and sediment conditions. Short flood pulses generate intense upstream erosion and downstream deposition. During the dry season, tidal currents erode the lower reach and transport sediment upstream, sustaining an approximate dynamic equilibrium throughout the year.

(6)

Flood characteristics in a MEMS are strongly shaped by channel morphology and tidal dynamics. Key bayonet nodes and channel curvature determine the spatial pattern of flood levels. Lowering shoal elevations and widening constricted bayonet zones produce the most effective reductions in flood levels.